A system based on the MPEG-4 (Moving Picture Experts Group Phase-4) which is currently in progress toward standardization in ISO/IEC JTC11/SC29/WG11 differs from a system based on ITU-T Recommendation H.263 in header information (an information signal for decoding) to be added to a coded bit stream which constitutes a transmitted signal.
FIG. 1(a) is a diagram showing a structure of an H.263 coded bit stream 201 based on the H.263 standard, into which header information 211 is multiplexed along with macroblock data 225 consisting of image coded data encoded according to the H.263 coding scheme. FIG. 1(b) is a diagram showing a structure of an MPEG-4 coded bit stream 202, into which header information 212 is multiplexed along with macroblock data 239 consisting of image coded data encoded according to the MPEG-4 coding scheme. As shown in these figures, they have different coded bit stream structures. In particular, the H.263 does not include header information about VO (Video Object), VOL (Video Object Layer), VOP (Video Object Plane) and the like, which are required for MPEG-4 decoding. Accordingly, to carry out the image communications based on the two schemes, separate image decoding apparatuses and image coding apparatuses are needed.
Here, it is not always necessary for a GOB start code 223 and GOB header information 224 in the H.263 coded bit stream 201, and resynchronization instruction code 237 and resynchronization information 238 in the MPEG-4 coded bit stream 202 to be inserted, but inserted as needed.
With such structures, the conventional coded bit streams present a problem in that an MPEG-4 compatible image decoding apparatus, for example, cannot decode the H.263 coded bit stream 201 generated according to the H.263 standard.
Furthermore, to decode the coded bit streams according to the MPEG-4 and H.263 standard, an image decoding apparatus must comprise two decoders based on the two schemes, which presents a problem of complicating the apparatus.
JP-9139937 already discloses a moving picture stream converting apparatus which is constructed by demultiplexer, a packetizer, and a multiplexer. The demultiplexer receives an MPEG 1 system stream including a stream of coded video data and a stream of encoded stream of encoded video data and a stream of encoded audio data and separates an elementary system (ES) of each of the video and audio data. A packetized elementary stream PES of MPEG 2 is formed by the packetizer from each of the video and audio elementary streams separated by the demultiplexer. The multiplexer divides the video and audio packetized elementary streams into transport packets each having a fixed length (188 bytes) and, thereafter, multiplexes the transport packets, thereby converting to a transport stream (hereinafter, referred to as “TS”) of MPEG 2. The MPEG 1 system stream as a conversion source is constructed by a plurality of packs, in which one pack obtained by combining a plurality of packs, in which one page obtained by combining a plurality of packets is set to one unit. A head pack is constructed by a pack header, a system header, and a plurality of packets. Each of second and subsequent packs is constructed by an ES header and an ES payload. A packed start code, a stream ID indicative of the king of video/audio and a channel number, a packet length of ES payload, time management information of a reproduction and time management information of a decoding have been stored as parameters in the elementary stream header. Video or audio encoded data has been stored in the ES payload. Each packet of the PES of MPEG 2 is constructed by a PES header and a PES payload. A packet start code, a stream ID indicative of the king of video/audio and a channel number, a packet length of PES payload, time management information of a reproduction output, and time management information of a decoding have been stored in the PES header. Video or audio encoded data has been stored in the PES payload.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide an image decoding apparatus capable of decoding the H.263 coded bit stream 201, an image coding apparatus for generating a coded bit stream decodable by the image decoding apparatus, and an image communications system and a coded bit stream converting apparatus for converting the H.263 coded bit stream to the MPEG-4 coded bit stream to carry out communication, all of which apparatuses have a simple structure.